Permanent magnet electrical machines are frequently used in hybrid vehicles as electrical drives. Permanent magnet synchronous machines include a rotor, in which the magnets generating the magnetic flux are usually situated. The magnets are relatively temperature sensitive, and may be permanently damaged at temperatures which, under certain circumstances, may be reached already at normal driving operation of a hybrid vehicle. In order to prevent this, the phase currents are usually limited beginning at the attainment of a critical magnet temperature.
Since the rotor is a revolving component part, and the temperature of the magnet is thus directly measured only with great effort, the magnet temperature is usually estimated from the measured stator temperature. In making this estimation of the magnet temperature, one assumes that the rotor and the permanent magnets are at approximately the same temperature as the stator. However, this estimation is subject to errors, particularly in response to transient processes.
In order to maintain a sufficiently great safety margin from a maximum permissible magnet temperature, the temperature threshold for the above-mentioned limitation of the phase current has to be selected to be relatively low. Consequently, the maximum performance of the machine cannot be completely utilized.